Non-noble metal catalysts for the oxygen reduction reaction

ABSTRACT

Non-noble metal transition metal catalysts can replace platinum in the oxidation reduction reaction (ORR) used in electrochemical fuel cells. A Ru x Se catalyst is prepared with comparable catalytic activity to platinum. An environmentally friendly aqueous synthetic pathway to this catalyst is also presented. Using the same aqueous methodology, ORR catalysts can be prepared where Ru is replaced by Mo, Fe, Co, Cr, Ni and/or W. Similarly Se can be replaced by S.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.10/630,634 filed Jul. 29, 2003, now allowed; which claims the benefit ofU.S. Provisional Patent Application No. 60/400,194 filed Jul. 31, 2002.Both of these applications are incorporated herein by reference in theirentireties.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The field of the invention relates to non-noble metal catalysts for theoxygen reduction reaction including methods of manufacture.

2. Description of the Related Art

Electrochemical fuel cells convert fuel and oxidant to electricity andreaction product. Solid polymer electrochemical fuel cells generallyemploy a membrane electrode assembly (“MEA”) in which an electrolyte inthe form of an ion-exchange membrane is disposed between two electrodelayers. The electrode layers are made from porous, electricallyconductive sheet material, such as carbon fiber paper or carbon cloth.In a typical MEA, the electrode layers provide structural support to themembrane, which is typically thin and flexible.

The MEA contains an electrocatalyst, typically comprising finelycomminuted platinum particles disposed in a layer at eachmembrane/electrode layer interface, to induce the desiredelectrochemical reaction. The electrodes are electrically coupled toprovide a path for conducting electrons between the electrodes throughan external load.

A significant emphasis has been placed to reduce MEA costs by reducingthe platinum loading while maintaining or even improving performance andreliability. One approach is to eliminate platinum entirely and replaceit with a cheaper alternative catalytic material. In particular, asignificant amount of work has been done working on replacing platinumfor the oxygen reduction reaction at the cathode.

Aside from cost, platinum catalysts have a further disadvantage whenused in direct methanol fuel cells (DMFCs) in which methanol is used asthe fuel. Namely, platinum at the cathode oxidizes methanol that crossesover from the anode leading to depolarisation and hence serious powerlosses in the cell.

Bron et al. (Journal of Electroanalytical Chemistry 500, 2001, 510-517)discloses a ruthenium-based catalyst for oxygen reduction. The catalystswere prepared by reacting Ru₃(CO)₁₂ with selenium for 20 hours indeaerated xylene under reluxing conditions. The product was filtered,washed with diethylether and dried in an oven at 90° C. to produce ablack powder. Bron et al. studied the effect of selenium and found amaximum benefit at about 15 mol % Se though catalytic activity was stillobserved in a selenium free catalyst. Bron concluded that the catalyticcenter in the selenium-containing catalyst is different from thecatalytic center in the selenium-free catalyst. Selenium was also foundto protect the catalyst against electrochemical oxidation and thereforeled to enhanced stability.

In a second publication produced by the same group, Tributsch et al.(Journal of Applied Electrochemistry 31, 2001, 739-748), found thatheating of this product resulted in the loss of carbon species in welldefined steps. The first step involved the loss of CO and CO₂ between250 and 350° C. and a second step was observed at temperatures above600° C. Further, Tributsch et al. found a loss of catalytic activityresulting from the release of carbon species at elevated temperatures.This led Tributsch et al. to propose a complicated catalytic structurecomprising a cubane-like organometallic ruthenium-complex on the surfaceof a ruthenium nanoparticle doped with a chalcogen (selenium or sulfur).Inspiration for this model appears to be an iron hydrogenase from theClostridium pasteurianum bacterium.

In a prior study on a related system, namely a MoRuS and MoRuSe system,Trapp et al. (J. Chem. Soc, Faraday Trans. 92(21), 1996, 4311-4319)arrived at significantly different conclusions. In the synthesis carriedout by Trapp et al., Ru₃(CO)₁₂ and Mo(CO)₆ were refluxed in xylene withsulfur or selenium for 20 hours. The catalyst powder was then filteredand dried at room temperature before being introduced into a tubularfurnace at 350° C. for one hour. Though Trapp et al. also performed aheating step, instead of reduced catalytic activity as reported byTributsch et al. supra, Trapp et al. observed improved activity. Infact, such heating step was referred to as “catalyst activation.” Inaddition, Trapp et al. concluded that the Ru species is the activecenter of the catalyst with some synergistic effects observed betweenthe ruthenium and the molybdenum sites in the mass-transport region.Trapp et al. also found that catalytic activity of the MoRuS was notaffected by methanol. Under conditions of simulated methanol cross-over,the activated MoRuS catalyst had a similar activity to platinum.However, similar activity was only observed with methanol present. Inthe absence of methanol, the activity of activated MoRuS catalyst wassignificantly worse than platinum.

Despite considerable efforts, a non-noble metal-based catalyst withactivity similar to platinum has yet to be developed. In addition,existing synthetic methodologies are directed to experimental scale and,as such, are not necessarily amenable to commercial scale production.For example, metal carbonyls, which are typically used as startingmaterials, are relatively expensive and typical solvent systems used,namely xylene, are toxic and environmentally damaging. Thus, even if thecatalysts were suitable for use in fuel cells, an environmentallyfriendly synthetic method would be needed.

The present invention fulfills these and other needs and providesfurther related advantages.

BRIEF SUMMARY OF THE INVENTION

In a first aspect of the present invention, a novel non-noble transitionmetal catalyst for the oxidation reduction reaction is prepared by:

dissolving selenium and Ru₃(CO)₁₂ in an organic solvent;

refluxing the organic solvent;

obtaining a precipitate; and

heating the precipitate to a temperature greater than or equal to 600°C. under an inert atmosphere.

In one embodiment, the organic solvent may be xylene. Furthermore, thetemperature for the heating step may be, for example, between 600 and700° C. Similarly, the heating step may be, for example, for more than10 hours or it may be for 12 hours. The inert gas may be, for example,nitrogen or argon.

The Ru_(x)Se catalyst thus prepared has an activity to the oxidationreduction reaction comparable to platinum such that it can be used atthe cathode in a polymer electrolyte membrane fuel cell. The Ru_(x)Secatalyst may be supported on, for example, carbon or unsupported.

In a second aspect of the present invention, the catalyst is preparedusing aqueous chemistry by:

dissolving a metal salt in an aqueous solution;

precipitating the metal;

introducing a chalcogen such as sulfur or selenium;

reacting the precipitated metal with the chalcogen by heating under aninert atmosphere.

If the metal salt is a ruthenium salt such as ruthenium (III) chloride,ruthenium (III) nitrate or ruthenium (III) acetate and the chalcogen isselenium, then a Ru_(x)Se catalyst as above will be synthesized.However, the aqueous methodology allows new non-noble transition metalcatalysts to be synthesized where the metal could be molybdenum, iron,cobalt, chromium, nickel and/or tungsten.

Precipitation of the metal may be done by reducing the metal with areducing agent such as sodium borohydride or formaldehyde.Alternatively, the metal may be precipitated with alkali, for exampleNaOH or NaHCO₃ to precipitate the corresponding metal hydroxide or metalcarbonate.

One method of introducing selenium is by dissolving selenium dioxide inthe aqueous solution and co-precipitating elemental selenium with thesame reducing agent as used for the precipitation of the metal.

Similarly, the catalyst thus prepared has an activity to the oxidationreduction reaction such that it can be used at the cathode in anelectrochemical fuel cell.

These and other aspects of the invention will be evident upon referenceto the attached drawings and following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plot illustrating the oxygen reduction current as a functionof the applied potential for an unsupported ruthenium-selenium catalyst.

FIG. 2 is a corrected Tafel plot for mass activity of theruthenium-selenium catalyst in FIG. 1 and a supported platinum catalyst.

FIG. 3 is a corrected Tafel plot for oxygen reduction catalystssupported on carbon in 0.5M sulfuric acid at room temperature comparingthe activities of platinum, ruthenium and ruthenium-selenium catalysts.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

A novel ruthenium-selenium (Ru_(x)Se) catalyst with catalytic activityfor the oxygen reduction reaction was synthesized (1) using an organicsolvent, namely xylene, and (2) using water as solvent. These catalystswere then tested for their catalytic activity for the oxygen reductionreaction useful in electrochemical fuel cells. The aqueous methodologyis also easily amenable to synthesizing a series of novel non-nobletransition metal chalcogen catalysts.

Organic Methodologies

Ru₃(CO)₁₂ may be used as the starting material for the Ru_(x)Se catalystand dissolved with selenium in an organic solvent such as xylene. Whileother suitable organic solvents are well known, the subsequentdiscussion will only refer to xylene. As selenium may not be readilysoluble in xylene, a selenium-xylene mixture may be refluxed for aperiod of time to effect dissolution before adding any Ru₃(CO)₁₂. Afterthe Ru₃(CO)₁₂ is added to the reaction mixture, the reaction mixture maybe refluxed again for a period of time to effect reaction betweenruthenium and selenium. A black precipitate may then be filtered andcollected. Complete reaction is not necessary nor required and unreactedselenium may be observed in the black precipitate.

The precipitate is then heated in a furnace under an inert atmosphere,such as, for example, nitrogen. Complete reaction between the rutheniumthe selenium occurs as well as activation to produce the Ru_(x)Secatalyst. In a specific embodiment, heating may be done at a temperaturegreater than or equal to 600° C., for example, between 600 and 700° C.

Aqueous Methodologies

As mentioned above, heating of the RuxSe catalyst completes reactionbetween the ruthenium and selenium and causes decarbonylation. However,it is unnecessary to begin with the relatively expensive startingmaterial of Ru₃(CO)₁₂ if the final step causes decarbonylation. Analternate synthesis involves the same heating of a ruthenium compound inthe presence of elemental selenium through a more environmentallyfriendly aqueous methodology.

The first step involves precipitation of a ruthenium salt in aqueoussolution. The ruthenium salt may be any ruthenium salt such as, forexample, ruthenium (III) chloride, ruthenium (III) nitrate or ruthenium(III) acetate. In a specific embodiment, the ruthenium salt is ruthenium(III) chloride, as it is the least expensive and the most readilyavailable ruthenium salt. Precipitation may be carried out with asuitable reducing agent such as, for example, formaldehyde or sodiumborohydrides to produce a metal precipitate. Alternatively, an alkalisolution, for example NaOH or NaCO₃, may be used to precipitateruthenium hydroxide or ruthenium carbonate, respectively.

In one embodiment, elemental selenium is produced by the addition ofselenium dioxide to the aqueous solution prior to reduction of theruthenium salt. In water, selenium dioxide dissolves to produce selenousacid, which in turn precipitates to the elemental selenium when reducedby, for example, NaBH₄.

After precipitation, the reaction mixture is filtered and heated in afurnace under an inert atmosphere, such as, for example, nitrogen. Theruthenium deposit decomposes and reacts with the elemental selenium toproduce the Ru_(x)Se catalyst. In a specific embodiment, heating may bedone at a temperature greater than or equal to 600° C., for example,between 600 and 700° C.

Not only does the above aqueous synthesis avoid both the use of thecostly starting material Ru₃(CO)₁₂, but it also avoids the use of atoxic and dangerous solvent system, namely refluxing xylene. As such,the above aqueous synthesis of a Ru_(x)Se catalyst is not onlyenvironmentally friendly but also amenable to large-scale commercialproduction.

Sulfur may also be used instead of or in addition to selenium asruthenium is known to react preferably with sulfur as compared toselenium (see, e.g., Trapp et al., supra). Precipitation of elementalsulfur in aqueous solution is likely to be impractical and, as such, amore specific method involves directly adding colloidal sulfur to theruthenium solution prior to precipitation of the metal so that theruthenium and sulfur are combined within a single powder. The colloidalsulfur can be produced from, for example, polysulfide alkalinesolutions. If sulfur is used as the chalcogen, hydrogen should beavoided in the heating step as sulfur reacts with hydrogen to producehydrogen sulfide. In contrast, selenium would not be expected toappreciably react with hydrogen.

In addition to substitution of the chalcogen, novel catalysts can besynthesized in which ruthenium is replaced by other non-noble transitionmetals such as molybdenum, iron, cobalt, chromium, nickel and/ortungsten. Without being limited thereto, examples of suitable saltswould include: ammonium molybdate, ammonium iron (III) citrate, ammoniumcobalt (II) sulfate hexahydrate, ammonium tungstate, and cobalt (II)nitrate hexahydrate. As with ruthenium, either one of selenium or sulfuror both may be used. Further, mixed catalytic systems wherein thecatalyst contains more than one non-noble transition metal may also besynthesized by dissolving and precipitating a mixture of at least twodifferent metal salts in the aqueous solution. While not being bound bytheory, the chalcogen appears to stabilize the transition metal suchthat it does not dissolve within the acidic environment of anelectrochemical fuel cell. This allows a greater variety of non-nobletransition metals to be used as catalysts for the oxygen reductionreaction.

EXAMPLE 1 Synthesis of Unsupported Ru_(x)Se with an Organic Solvent

0.15 g Se was added to 100 ml xylene and refluxed under bubblingnitrogen overnight before being allowed to cool to room temperature.2.85 g Ru₃(CO)₁₂ was then added to the reaction mixture and refluxedunder nitrogen for a further 20 hours. A black precipitate was thenwashed and dried. On grinding, the black precipitate was found tocontain reddish brown streaks that was presumed to be unreactedelemental selenium. The material was then heated under nitrogen to 600°C. in a quartz tube furnace for 12 hours. After heating, the Ru_(x)Sepowder was completely black without any reddish brown streaks therebyindicating complete reaction.

EXAMPLE 2 Synthesis of Carbon Supported Ru_(x)Se with an Aqueous Solvent

1.0361 g Vulcan XC72R carbon was added to 1l water in a 4 l beaker.0.4034 g RuCl₃ and 0.1071 g SeO₂ were dissolved in 500 ml water andsubsequently added to the 4 l beaker. Wetting was assured by adding 100ml propan-1-ol and then stirred at 80° C. for 1 hour. The mixture wasthen allowed to cool to room temperature. A 1 l solution of 0.1M NaBH₄in 0.2M NaOH was added to the beaker and allowed to react slowly. ExcessNaBH₄ was removed by heating to 80° C. for 5 minutes and cooling. Thepowder was then filtered and washed in water and dried at 80° C.overnight. The powder was then placed in a quartz lined tube furnaceunder nitrogen and heated at 15° C. min⁻¹ to 600° C. and held at 600° C.for 2 hours. The catalyst was then removed from the furnace, cooled andground to a fine powder.

EXAMPLE 3 Synthesis of Carbon Supported Ru Catalyst

The same methodology for synthesizing the RuxSe catalyst of Example 2was employed except that no SeO₂ was added to the reaction mixture.

EXAMPLE 4 Preparation of Electrode for Testing Oxygen Reduction ReactionActivity

The catalyst powders were tested for oxygen reduction reaction activity.Catalyst powder was dispersed in glacial ethanoic acid and a portionthereof deposited onto a clean gold mesh electrode. After drying with astream of warm air, the electrode was then placed in a standard threeelectrode cell containing 0.5M H₂SO₄ as electrolyte, a gold wire counterelectrode and a reversible hydrogen reference electrode. After bubblingwith oxygen gas to saturate the acid, the potential was swept at 5 mVs⁻¹from 1.0V to 0.1V vs RHE to give the oxygen reduction current as afunction of the applied potential.

FIG. 1 illustrates the oxygen reduction current as a function of theapplied potential for the unsupported Ru_(x)Se catalyst. To compare theactivity of the unsupported Ru_(x)Se catalyst with platinum, a Tafelplot was prepared as in FIG. 2. In both of FIGS. 1 and 2, λ is used toindicate the unsupported Ru_(x)Se catalyst and σ indicates a baselinemeasurement of a platinum catalyst supported on XC72R carbon supplied byJohnson Matthey Inc. As the platinum was supported by carbon and theRu_(x)Se catalyst was unsupported, the results as illustrated in FIG. 2are normalized by mass of actual metal present. FIG. 2 indicates that ona mass basis, the Ru_(x)Se catalyst is comparable in activity asplatinum for the oxygen reduction reaction. Further observations of theRu_(x)Se catalyst indicated that the particle size was very small, i.e.,less than 50 nm in diameter. Further, no dissolution of the Ru_(x)Secatalyst was observed during electrochemical experiments as may beexpected with elemental ruthenium. Without being bound by theory, thisindicates an increased stability of the catalyst as compared toelemental ruthenium. From the amounts of starting material used, theunsupported Ru_(x)Se catalyst would have a Ru:Se ratio of approximately7:1.

FIG. 3 illustrates a corrected Tafel plot for oxygen reduction catalystssupported on carbon in 0.5M sulfuric acid at room temperature comparingthe activities of supported Ru_(x)Se catalyst with platinum andelemental ruthenium catalysts. Plot A illustrates the results obtainedfor 40% Pt on XC72R carbon (supplied by Johnson Matthey) whereas plot Bis for 40% Ru on XC72R carbon and plot C is for 40% Ru_(x)Se on XC72Rcarbon. FIG. 3 also illustrates two key features in addition to thepoints raised above with respect to the unsupported Ru_(x)Se catalyst.First, FIG. 3 is a more accurate comparison between the activities ofthe catalysts as compared to FIG. 2 as all catalysts are supported. Theactivity of the supported Ru_(x)Se catalyst is shown to approach theactivity of the supported platinum catalyst and represents animprovement in activity and stability as compared to the supported Rucatalyst. Second, the aqueous methodology used to make the Ru_(x)Seproduces an active catalyst in a relatively environmentally friendly andcost effective manner.

While particular steps, elements, embodiments and applications of thepresent invention have been shown and described, it will be understood,of course, that the invention is not limited thereto since modificationsmay be made by persons skilled in the art, particularly in light of theforegoing teachings. It is therefore contemplated by the appended claimsto cover such modifications as incorporate those steps or elements thatcome within the spirit and scope of the invention.

1-6. (canceled)
 7. A non-noble transition metal catalyst prepared by:dissolving selenium and Ru₃(CO)₁₂ in an organic solvent; refluxing theorganic solvent; obtaining a precipitate; and heating the precipitate toa temperature greater than or equal to 600° C. under an inertatmosphere.
 8. The catalyst of claim 7 wherein the catalyst issupported.
 9. An electrochemical fuel cell comprising a non-nobletransition metal catalyst at the cathode wherein the catalyst isprepared by the method of claim
 7. 10-22. (canceled)
 23. A non-nobletransition metal catalyst prepared by: dissolving a metal salt in anaqueous solution, the metal is ruthenium, molybdenum, iron, cobalt,chromium, nickel or tungsten; precipitating the metal; introducing achalcogen, the chalcogen being sulfur or selenium; and reacting theprecipitated metal with the chalcogen by heating under an inertatmosphere.
 24. The catalyst of claim 23 wherein the catalyst issupported.
 25. An electrochemical fuel cell comprising a non-nobletransition metal catalyst of claim 23 at the cathode.
 26. Anelectrochemical fuel cell stack comprising at least one electrochemicalfuel cell of claim 24.